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Carbon Monoxide (Blood)

Does this test have other names?

CO blood test, CO blood gas, arterial blood gas

What is this test?

This test measures the level of carbon monoxide (CO) in your blood.

Carbon monoxide is a colorless, tasteless, and odorless gas made by combustion. Breathing in CO can be fatal because it doesn't allow oxygen to get to your heart and other organs.

This test looks for carboxyhemoglobin. This substance is created in your blood when hemoglobin combines with carbon monoxide instead of oxygen. 

Most deaths from CO result from smoke inhalation. Other sources include malfunctioning heaters, improperly ventilated kitchen stoves, improperly vented tools, camping stoves, charcoal grills, water heaters, and cars with their engines running in an enclosed space like a garage. All of these can cause CO to spread throughout a building.

Why do I need this test?

You may need this test if your doctor suspects that you have CO poisoning. Symptoms of carbon monoxide poisoning include:

  • Headache

  • Nausea

  • Dizziness

  • Weakness

  • Diarrhea

  • "Cherry red" skin and lips

Severe poisoning can produce nerve system symptoms, such as:

  • Seizures

  • Coma

Carbon monoxide poisoning can be more difficult to identify in very young children than in adults. For example, a child may merely appear fussy and not want to eat.

You may also have this test if you have been exposed to CO, especially if you inhaled smoke during a fire. You may also have this test if you've been near a car that's had its engine running in an enclosed space for an extended amount of time.

What other tests might I have along with this test?

Your doctor may also order these tests:

  • Toxicology screen

  • Chest X-ray

  • Pregnancy test in women, because CO exposure puts the fetus at high risk for problems

Your doctor may also order an MRI scan if you also have symptoms of nervous system problems.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Results are given as a percentage or as a decimal. These are the normal ranges for CO levels in the blood:

  • Adults:  less than 2.3 percent, or 0.023

  • Adult smokers: 2.1 to 4.2 percent, or 0.021 to 0.042

  • Adult heavy smokers (more than two packs a day): 8 to 9 percent

  • Hemolytic anemia: Up to 4 percent

  • Newborn: greater than 12 percent

If your levels are higher, you may have CO intoxication or poisoning.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Other factors aren't likely to affect your results.

How do I get ready for this test?

You don't need to prepare for this test. But be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.